The cytoplasmic site of gene expression and use of virally encoded enzymes is a distinguishing feature of vaccinia virus and other poxvirus vector systems that contributes to their consistent ability to express foreign genes derived from a variety of prokaryotic, eukaryotic, and viral sources. This feature, together with their ability to stably integrate and package large amounts of additional DNA without loss of infectivity, their wide host range, and the development of methods for isolating recombinant viruses, account for their diverse use and popularity. We have developed the highly attenuated and host restricted MVA strain of vaccinia virus as an expression vector because of the added safety. Cotton rats inoculated with recombinant MVA expressing the parainfluenza 3 (PIV3) F or HN glycoproteins by intramuscular or intranasal routes produced high levels of antibody. Expression of HN was more effective in inducing immunity to PIV3 challenge, reducing viral titers in the nasal turbinates by at least 4.7 logs and in the lungs by 3.4 logs, similar to that achieved by immunization with PIV3. These studies support further testing of recombinant MVA/PIV3 viruses as safe and effective candidate vaccines. The potential use of recombinant vaccinia virus for cancer therapy is being evaluated in model murine systems. We have shown that cytokines and co-stimulatory molecules can enhance the therapeutic effects. Favorable results were obtained with IL-12 and B7-1. The vaccinia virus/bacteriophage T7 expression system was adapted to Chinese hamster ovary (CHO) cells because of the use of these cells for synthesis of recombinant proteins in research and biotechnology. We constructed a recombinant vaccinia virus with an intact cowpox virus hr gene to overcome the host restriction, in addition to the T7 RNA polymerase gene. With this virus, synthesis of a recombinant protein occurred in CHO cells at the level observed in permissive cell lines.